1. Field of Invention
This invention relates to strengthening the reinforced concrete elements to increases the elements static and dynamic load capacities, and its ductility without significantly increasing the dimensions or weights of these elements, or even harming the concrete section. The invention also includes the apparatus used to apply the required confining pressure.
2. Description of Prior Art
Various techniques are available for strengthening the structural elements whether by making steel or concrete jacketing or in the case of columns by encompassing the existing reinforced concrete section with masonry blocks or strengthening it by either carbon or glass fiber reinforced plastic reinforcement. Through surveying these techniques, some undesirable effects have been found. Summary of these techniques with its disadvantages are presented herein:
1) Concrete Jacketing
Concrete jacketing has been widely used in repairing, strengthening, and improving the ductility capacity of damaged and existing reinforced concrete columns. But this technique increases dimensions of the structural element to an undesirable extent. The extra weight resulting from the concrete jacketing may lead to problems in foundations and the underlying soils. In addition, concrete jacketing is not suitable for strengthening in high rise buildings.
2) Steel Jacketing
Circular and rectangular steel jacketing are usually used to increase the flexural strength, ductility, and shear capacity of a part in the column. However, this technique mainly provides some local strength capacity increase. It also needs special equipment, besides assembling the steel jacket without post-tensioning it on the reinforced concrete member. Therefore, the clearance between the steel jacket and the reinforced concrete element permits internal strains to take place in the original reinforced concrete section.
3) Masonry Block Jacketing Similar to the concrete jacketing method, a masonry block jacket can be used for repairing and strengthening of existing and damaged columns. The undesirable effects are mainly the increase in dimensions and the additional over loads added to the existing reinforced concrete columns. Besides, this technique is only used in low to medium rise reinforced concrete buildings.
4) Partial Masonry Infill
This technique have been used for increasing the stiffness and strength of structures to control story displacements from high wind loads and other natural forces including seismic loads. An architectural disadvantage of using an infill wall retrofit for an existing building is the loss of space and access near the wall, along with adding more loads to the reinforced concrete buildings.
5) Strengthening the Reinforced Concrete Elements Using Either Carbon or Glass Fiber Reinforced Plastic Reinforcement
This technique has been used for increasing the stiffness and strength of structural elements like columns, beams, and slabs. The main disadvantage of this technique arises when the strengthened element is subjected to a high temperature at which the strengthening material and epoxy used to bond them lose a great part of their strength. One more disadvantage is the brittle behavior of either carbon or glass fiber reinforced plastic materials, which decreases the ductility of the strengthened reinforced concrete element. At high loads, just before failure, the concrete cover is spilled away with these strengthening materials resulting in a sudden failure in the element.
Al-Tuhami and Sakr 1998, suggested an idea to strengthen the reinforced concrete columns. Their idea hypothesizes include making grooves in the original reinforced concrete cover to embed longitudinal bars with Epoxy bond materials between new steel bars and concrete. Then attach pre-stressed spiral or tied stirrups around the column. Finally adding cement mortar to cover the new bars and stirrups. The disadvantages of this idea are the harming of the original reinforced concrete section that arises from making grooves in the concrete cover which can lead to compression failure of the column. In addition the authors did not define how they can attach the pre-stressed spiral or tied stirrups to the concrete section.
U.S. patent application Ser. No. 07/646,288 to Fyfe (1991) disclose a limited method to improve the strength of a concrete column, supporting an overhead load and having a base end resting on a surface, using stretchable fibers. The fibers overwrapped about the surface of the column. Then applying a coat of hardenable material over the layer of the fibers. Afterwards a quantity of a hardenable liquid is injected under the layer of the fibers and over the surface of the work area to cause the fibers to undergo more stretching. The main disadvantage arises when the strengthened element is subjected to a high temperature at which the strengthening material and epoxy used to bond them lose a great part of their strength. In addition, this technique is limited to certain column configurations.
U.S. patent application Ser. No. 07/036,101 to Creedon (1988) shows a complex method for forming prestressed concrete members using casing disposed around the outside surface of the concrete member and is spaced therefrom so that a cavity is formed between the casing and the outside surface of the concrete member. Then a pressurized medium is injected into the cavity between the casing and the concrete member with predetermined pressure. It can be seen that, this technique needs complex apparatus, beside the difficulty of using this method to strengthen the existing concrete members especially for non-circular shaped cross-sections.
Accordingly, several objects and advantages of my strengthening technique are:
1. It can avoids large dimensions of the strengthened elements compared with other techniques like reinforced concrete and masonry block jackets, which saves more space.
2. The increase of column weight in the present method is so small compared with weights added in case of using concrete or masonry jackets.
3. The method provide increases the static load capacity for existing reinforced concrete elements which is the aim of the most available strengthening techniques.
4. One important object of the present technique is to increase the seismic durability of the reinforced concrete elements especially columns, i.e. the strength of column against long term shaking will increased which cannot be achieved using either carbon or glass fiber reinforced plastic reinforcement methods during strong shaking.
5. It provides a very simple strengthening process and can be carried out so quickly.
6. Another object of the present method is to reduce the strengthening and repairing costs.
7. The present method can solve the problems of discontinuities in connections resulting from concrete, masonry block jacketing, and partial masonry infill techniques.
8xe2x80x94By the invention, the achieved strength of the reinforced concrete strengthened element is attained instantly and does not require the setting time needed in case of reinforced concrete jacket.
Further objects and advantages of my invention will become apparent from a consideration of the drawings and ensuing description.